Thermoforming apparatus for moving sheet material

ABSTRACT

A method and apparatus for conveying a sheet of thermoplastic material at least through the heating zone of a thermoforming machine in which the sheet margins are attached to laterally spaced conveyors by pressing and permanently deforming portions of the sheet margins into longitudinally spaced openings in the conveyors, prior to advance of the sheet through the heating zone. A sheet forming press is operated in timed relation with the conveyors to press and permanently deform portions of the sheet margins into the openings in the conveyor.

United States Patent Desnick et al.

[ Feb. 15,1972

[54] THERMOFORMING APPARATUS FOR MOVING SHEET MATERIAL [72] Inventors: Mandel L. Desnick, St. Louis Park; Jesse A. Willcox, Excelsior; Jay A. Christensen,

Bloomington, all of Minn.

[73] Assignee: Anderson Bros. Mfg. C0., Rockford, Ill.

[22] Filed: May 26, 1970 [21 Appl. No.: 40,676

[52] US. Cl. ..425/343, 425/DlG. 39, 264/322, 425/405, 425/412 [51] Int. Cl ..B29c 17/03 [58] FieldofSearch ..l8/4B,5A, 19 R, 19 P, 19 F, 18/35, DIG. 39; 264/322 [56] References Cited UNITED STATES PATENTS 2,282,423 5/1942 Kopithe ..l8/l9 F 2,684,104 Dessart et al ..18/19 R X 3,193,881 7/1965 3,228,066 1/1966 3,359,600 12/1967 O'Brien et a1 ..l8/DlG. 39

Primary Examiner-R0bert L. Spicer, Jr. AttorneyMcCanna, Morsbach, Pillote & Muir [5 7] ABSTRACT 18 Claims, 11 Drawing Figures PAFENFEBFEB 15 I972 SHEET 1 [1F 3 THERMOFORMING APPARATUS FOR MOVING SHEET MATERIAL BACKGROUND OF THE INVENTION Many thermoplastic sheet materials tend to undergo dimensional changes during heat softening of the materials preparatory to forming of the sheet material. Some thermoplastic materials are oriented or prestretched and tend to shrink upon heating and even those which are not prestretched or oriented, frequently contain strains which tend to change upon heating and cause dimensional variations.

In the feeding of thermoplastic sheet material through a thermoforming apparatus, it is desirable to firmly retain the lateral margins of the sheet material to inhibit shrinking and sagging during heat softening and subsequent forming of the sheet material. It has heretofore been proposed to use a pair of endless conveyors having sharpened teeth or spikes which impale the marginal edges of the sheet to hold the sheet margins on the conveyor during advance through the heating zone of the thermoforming apparatus. Such apparatus requires that the teeth be maintained sharp in order to facilitate penetration of the sheet, and the penetration of the sheet by the teeth or spikes tends to weaken the sheet along the margins. It has also been proposed to provide plastic sheet conveying mechanism which clamps or grips the marginal edges of the sheet to hold the edges during advance through the thermoforming ap paratus. In one clamp-type sheet conveying apparatus heretofore used, a plurality of individually operable clamp members are carried by a pair of endless conveyors and the individual clamp members are actuated to clamp opposite faces of the marginal edges as the sheet is advanced into the thermoforming machine and to release the clamps as the sheet exits from the thermoforming machine. In another form of plastic sheet conveying mechanism a pair of opposed conveyors are provided at each margin of the sheet and arranged so as to clamp the sheet margins between the adjacent runs of each pair of opposed conveyors. Such clamptype conveyor apparatus require that the clamping pressure be maintained on the sheet margins during advance of the sheet through the thermoforming apparatus and are relatively complex. Moreover, in the clamp-type sheet conveying apparatus, difficulties are encountered in maintaining uniform clamping pressure along the margins of the sheet with the resultant tendency towards distortion and wrinkling of the sheet edges.

SUMMARY OF THE INVENTION In the present invention, the margins of the sheet of thermoplastic materials are attached to spaced endless conveyors by forming protuberances in the sheet margins that extend into longitudinally spaced openings in the endless conveyors. The protuberances are formed in the sheet margins by pressing portions of the sheet margins into openings in the laterally spaced endless conveyors and, preferably, the protuberances are formed with heads sufficiently larger than the minor cross section of the openings in the conveyors to inhibit separation of the sheet margins from the conveyors. The endless conveyors are preferably in the form of thin, flat endless bands having the sheet engaging faces thereof disposed generally parallel to the plane of the sheet to provide a smooth continuous sheet support and to provide relatively high resistance to lateral movement in the plane of the sheet, to thereby minimize shrinking and distortion of the sheet during heat softening and forming of the same. The sheet margins maybe preheated prior to pressing portions of the same into the openings in the conveyor.

An improved sheet press is provided for forming the protuberances in the sheet and includes a clamp die for clamping the sheet margins to each conveyor and a resilient press head movable in the clamp die and laterally confined thereby for pressing portions of the sheet margins into the openings in each conveyor. A fluid actuator is connected to the clamp die and press head and arranged to sequentially move the clamp die into engagement with the sheet and thereafter move the press head relative to the clamp die to form protuberances in the sheet. I

An important object of this invention is to provide an improved method and apparatus for conveying sheet 'thermoplastic material through a thermoforming apparatus, wherein the sheet margins are supported and retained against lateral movement without significant weakening or wrinkling of the sheet margins.

Another object of this invention is to provide a method and apparatus for conveying sheet thermoplastic material wherein the sheet margins are attached to spaced endless conveyors without rupturing and weakening the web margins.

Yet another object of this invention is to provide a method and apparatus for conveying thermoplastic sheet material in which the sheet margins are attached to spaced endless conveyors by forming protuberances in the sheet margins that extend into openings and interlock with the conveyors.

Still another object of this invention is to provide an improved sheet press apparatus for forming protuberances in a sheet of thermoplastic material.

These, together with other objects and advantages of this invention, will become apparent from the following description when taken in conjunction with the drawings wherein:

FIG. 1 is a top plan view of a thermoforming apparatus embodying the improved sheet conveyor apparatus of the present invention;

FIG. 2 is a side elevational view of the apparatus shown in FIG. 1;

FIG. 3 is a fragmentary perspective view illustrating the sheet press apparatus for attachingthe sheet margins to the endless conveyor;

FIG. 4 is a sectional view through the sheet press apparatus showing the same in its open position;

FIG. 5 is a fragmentary transverse sectional view through the sheet press apparatus taken on the plane 5-5 of F IG. 4;

FIG. 6 is a fragmentary sectional view through the sheet press apparatus illustrating the sheet press in its closed position;

FIG. 7 is a fragmentary sectional view taken on the plane 77 of FIG. 6 and showing the parts on an enlarged scale;

FIG. 8 is a fragmentary sectional view through the heating zone taken on the plane 88 of FIG. 1;

FIG. 9 is a fragmentary perspective view illustrating the apparatus for separating the sheet margins from the endless conveyors;

FIG. 10 is a fragmentary longitudinal sectional view taken on the plane l0l0 ofFlG. 9; and

FIG. 11 is a diagrammatic view illustrating the electrical and fluid controls for the forming and conveying apparatus.

The sheet conveying apparatus is shown in FIGS. 1 and 2 0f the drawings incorporated into a thermoforming apparatus for thermoplastic sheet material. The thermoforming apparatus in general includes an elongated frame 21 having a means 22 adjacent one end of the frame for supplying a sheet S of thermoplastic material to the forming apparatus; a heating station 23 for heat softening the sheet of thermoplastic material; and a forming station 24 for forming andshaping the heat-softened thermoplastic sheet. In the embodiment shown, the thermoplastic material is supplied from a roll R to the thermoforming machine, it being understood that the sheet of thermoplastic material can be fed directly to the thermoforming machine from an extruder, if desired. As is well understood in the art, the thermoforming machine can employ additional stations along its length including filling stations, cover applying stations, sealing stations and trimming stations, if desired.

The sheet conveying apparatus includes a pair of endless conveyors 31 extending from adjacent the inlet of the machine at least through the heat softening and forming stations 23 and 24 and preferably extending throughout the length of the thermoforming machine including any additional stations which may be provided thereon. The endless conveyors can be of the articulated link type but are preferably in the form of a pair of laterally spaced endless tapes or bands supported as by wheels 32 and 33 adjacent opposite ends of the thermoforming machine. As shown in FIG. 2, mechanism is provided for driving one pair of the wheels 33 and the other pair of wheels 32 are mounted in shiftable bearing blocks 34 to enable adjustment of the tension on the endless conveyors 31. The shiftable bearing blocks v34 are conveniently yieldably urged as by springs 35 in a direction to maintain tension on the endless bands 31.

In order to provide a dwell cycle for the forming operation, the conveyor drive mechanism 38 is arranged to intermittently drive the conveyor to advance the same, and to then allow the conveyors to dwell for a period sufficient to accommodate the forming cycle of the thermoforming apparatus 24. Any conventional drive mechanism can be employed and, in the embodiment disclosed, there is provided a one revolution clutch 39 which is selectively operable to connect the drive motor (not shown) to a drive shaft 41 to drive a crank 42 through one revolution. The crank 42 is connected through a link 43 to a crank 44, and which crank 44 is connected through a oneway clutch (not shown) to the conveyor drive shaft 45 to drive the conveyors through one-half of the revolution of the drive shaft 41 and to allow the conveyor to dwell during the other half revolution of the drive shaft. The link 43 can be adjustably connected to the crank 42 to enable adjustment of the distance through which the conveyors are advanced'during each rotation of the shaft 41. The endless conveyors 31 extend around the wheels 32 and 33 and may be driven as by frictional engagement of the endless conveyors with wheels 33 or in more positive fashion by pins or lugs (not shown) on the wheels 33 which extend into longitudinally spaced openings in the endless conveyors.

The wheels 32 and 33 support the endless conveyors 31 with their upper runs generally parallel and the sheet thermoplastic material is supplied to the upper runs of the conveyors with the marginal edges overlying the conveyors. An infeed strip alignment device 22a is provided to laterally guide the web S as it enters the machine to align the web margins with the endless conveyors. The conveyors 31 have longitudinally spaced openings 31a formed therein and mechanism is provided for forming protuberances in the sheet margins which extend into the openings 31a to attach the sheet margins to the conveyors for advance therewith through the thermoforming apparatus. In the preferred form openings 31a are formed at longitudinally spaced locations in a continuous endless band or strip. However, it is contemplated that the openings 31a could be formed in plates or links formed as a part of or attached to an articulated link-type conveyor chain.

The sheet margins are attached to the conveyors by sheet presses 50 located in advance of the heating station and arranged to press and permanently deform portions of the sheet margins into the openings 31a in the respective endless conveyors 31. The sheet presses each include an anvil 51 underlying the upper run of the conveyor bands 31 and a sheet clamping and pressure applying head 52 which overlies the anvil and which is operated in timed relation with the advance of the conveyor to, clamp portions of the sheet margins to the conveyors and press the same into the openings in the conveyors, while the conveyors are dwelling. The sheet presses 50 may be mounted on the frame 21 of the thermoforming apparatus in any suitable manner and, as best shown in FIG. 3, the press units each include a generally Cshaped frame structure 55 having a fixed platen 56 at the lower leg thereof which underlies the upper run of the conveyor bands 31 and which has a means such as a groove 57 for receiving the anvil 51. A clamp and press head actuator 58 is mounted on the upper leg of the C-shaped frame structure 55 and is operatively connected to the clamp and pressure applying head to actuate the latter. In the preferred embodiment illustrated, the actuator 58 includes I a cylinder 61 attached to the upper leg of the C-shaped frame structure. The cylinder is closed at its upper end 61a and is opened at its lower end and'a piston 62 is slidable in the cylinder. Seal means such as an O-ring 63 and wiper 64 is provided to form a sliding seal between the piston and cylinder and a movable platen 65 is attached as by fasteners 66 to the lower end of the piston. Fluid under pressure is supplied through an inlet passage 68 in the upper end of the cylinder to move the clamp and pressure applying head in a working stroke and, as shown, the piston has a relatively large crosssectional area so as to provide a relatively high-working force for the clamp and pressure applying head. The actuator 58 also includes a fluid operated means for retracting the piston, with a relatively lower force than that used for extending the same. As best shown in FIGS. 4 and 6. the piston 62 has a cylinder 71 formed therein and a piston 72 is disposed in the cylinder and connected through a rod 73 to the upper leg of the C-shaped frame structure 55. The rod 73 is fixed to the frame structure 55 and, as best shown in FIG. 4, is threaded in the stationary cylinder 55 and locked in position by a nut 73a. The rod 73 extends through a gland 75 on the piston 62 and the piston 72 has seal means such as O-rings 76 to form a sliding seal with the cylinder 71. Fluid under pressure is applied to the cylinder 71 in the piston 62 to raise the piston after the completion of the working stroke thereof. In the form shown, fluid pressure is applied through a passage 78 in the rod 73, which passage has lateral openings 78a that communicate with the cylinder 71 above the piston 72. As will be seen, the effective area to which fluid pressure is applied for retracting the piston is equal to the difference between the area of the cylinder 71 and the cross-sectional area of the rod 73, which effective area is small as compared to the area at the upper' end of the piston 62 to which fluid pressure is applied for extending the piston. With this arrangement, the fluid actuator is operative to apply a relatively high force for clamping and forming the sheet, and a relatively lower force is provided for retracting the clamp and forming head. A resilient stop 61b is provided inside the cylinder 61 to limit upward movement of the piston 62.

The clamp and forming head 62 includes a clamp die 81 which is connected as by guide rods 82 to a block 83. The block 83 is secured to the platen 65 as by fasteners 84 and the guide rods 82 slidably extend through openings 85 in the block to support the clamp die on the block for limited lost motion relative thereto. Springs 86 are interposed between the block 83 and clamp die 81 and normally urge the clamp die away from the block to a position as shown in FIG. 4 in which heads 82a on the guide rods engage stops 850 on the block 83.

The clamp die 81 has a cavity 810 therein and clamp faces 81b that extend around the margin of the cavity at its lower end for engaging the marginal portion of the sheet S to clamp the same to the anvil 51 and platen 56. As best shown in FIGS. 5 and 7, the cavity 81a has a width to span the openings3la in the respective conveyor bands so that the clamp die is operative to clamp the sheet margins to the conveyor band along opposite sides of the openings 31a. The springs 86 are arranged to yield after the clamp die engages the sheet, to limit the pressure applied by the actuator to the clamp die, and the strength and number of springs is selected to apply the desired clamping pressure.

Provision is made for applying pressure to the sheet margin within the cavity 81a of the clamp head to press and permanently defon'n portions of the sheet margins into the openings in the conveyor bands. It is contemplated that pneumatic pressure may be used for some plastic sheet material to press and permanently deform portions of the sheet margins into the openings in the conveyors 31 and that pneumatic pressure could be applied to the die cavity 81a for this purpose. However, many plastic sheet materials require relatively high-forming pressures to permanently deform portions of the sheet margins into the relatively small conveyor openings 31a. Accordingly, in the preferred embodiment the pressure applying means has a pressure applying head 91 formed of a resilient elastomeric material such as rubber. The resilient pressure applying head 91 is dimensioned to closely fill the cavity 81a in the clamp die so that, when pressure is applied to the resilient pressure applying head, it acts like a confined fluid to transmit the pressure to the sheet margins and press portions of the sheet margins into the conveyor openings 3141. Pressure is applied to the resilient head 91 through a rigid pressure distributing member 92. As shown, the pressure-distributing member 92 has a channel-shaped cavity for receiving the upper end of the resilient pressure-applying head 91 and is slidably supported within the cavity 81a. Pressure applying rods 93 are connected at their lower end to the distributing member 92 and the rods slidably extend through openings 94 and 95 in the clamp die 81 and block 83 respectively. The openings 95 in the block 83 have an enlarged upper portion 95a defining a shoulder 95b intermediate the ends of the opening and heads 93a are provided on the rods 93 and arranged to engage the respective shoulder to limit downward movement of the pressure applying head relative to the clamp die. Adjustable upper stops 95c are adjustably mounted in the enlarged upper portion of the openings 95 and arranged to engage the heads 93a on the pressure-applying rods 93 during a working stroke of the actuator 58 to positively urge the rods and pressure applying head downwardly. As shown in FIG. 4, the heads 93a normally engage the stops 95b to support the pressure-applying head 91 with its underside spaced a slight distance above the clamping faces 81b on the clamp die. When the actuator moves the block 83 downwardly in a working stroke, the clamp die initially engages the margin of the thermoplastic sheet and the springs 86 are compressed until the heads 93a on the rods 93 engage the upper stops 950. At that time, the pressure head 91 is positively moved downwardly by the actuator 58 and the clamp die is yieldably retained in clamping engagement with the sheet by the springs 86. As the resilient head 91 is forced downwardly, it presses the portions of the sheet that overlie the openings 31a in the conveyors into the openings to form protuberances in the sheet margins which extend through the openings and attach the sheet margins to the endless bands. As best shown in FIG. 7. the openings 310 are preferably rounded or at least beveled to minimize sharp comers and to also facilitate expansion of the protuberances adjacent the underside of the openings to provide an enlarged head on the protuberances which retains the sheet margin on the conveyor and inhibits lateral separation. The conveyor bands are relatively thin and in order to improve attachment of the sheet margins to the conveyor bands, the anvil 51 is advantageously formed with a groove or channel 51a in the region underlying the openings 31a in the conveyors and of a depth sufficient to allow the protuberances to extend through the endless bands 21 short distance. The length of the pressure-applying head 91 is selected so as to attach a length of the sheet margins to the conveyors 31 at least equal to the distance the conveyor is advanced during each cycle of the conveyor. While a single pressure-applying head 91 is herein illustrated for pressing portions of the sheet margins into a plurality of conveyor openings 31a, it is apparent that the pressure applying head could be made in the form ofa plurality of individual pressure applying members arranged in a row to register with individual ones of the conveyor openings 3 la, with the individual pressure applying heads operated by a common actuator 58 or by individual actuators, if desired.

It is contemplated that the method and apparatus for conveying thermoplastic materials can be used with any thermoplastic material that can be permanently deformed, with'or without application of heat, into a shape sustaining form. However, the size of the conveyor openings should be correlated with the type and thickness of the plastic sheet material used to assure that the sheet material, when pressed into the openings, will have sufficient rigidity to inhibit withdrawal of the protrusion from the opening. Thus, for very thin thermoplastic films such as those of the order of less than mils in thickness it is generally necessary to use relatively smaller openings spaced and somewhat closer together than with thicker sheet thermoplastic material. Many thermoplastic sheet materials can be cold-formed at temperatures substantially below their softening temperatures, upon the application of sufficient pressure, to form the desired protuberances in the sheet margins for attaching the sheet margins to the tapes. However, in order to adapt the conveying mechanism for use with a wide range of thermoplastic sheet materials including those which are formable only when heated, apparatus is provided which is selectively operable to preheat the marginal edges only of the thermoplastic sheet material, prior to attaching the sheet margins to the conveyors. As shown in FlGS. 1-3, heating bars 101 are mounted, as by hinge-type brackets 102, on the frame 21 of the thermoforming apparatus for movement into and out of the positions shown inFlG. 3, overlying the marginal edges of the sheet. As shown, handles 104 are provided to facilitate movement of the heating bars into and out of operative position. The bars are heated as by heating element 101a disposed in the bars and which is controlled by an adjustable thermostat to enable selective adjustment of the temperature of the bars and hence the heating of the thermoplastic material. For those materials which are cold-formable and which require no preheating, the heating bars can be moved to an inoperative position spaced substantially away from the thermoplastic sheet material.

Guide means are provided for laterally and vertically guiding the endless band conveyors 31. As shown, opposed pairs of grooved rollers and 111 are mounted as by brackets 112 on the frame adjacent the inlet and outlet ends thereof. The grooved rollers engage opposite marginal openings of the respective endless band 31 and vertically and laterally support the same. The bands themselves are quite resistant to distortion in a direction paralleling the plane of the sheet material and minimize the amount of lateral guiding required for the endless bands intermediate the ends of the conveyor. In order to minimize or avoid heat softening of the marginal edges of the sheet during advance through the heating zone 23, heat shields are provided for the endless conveyors and sheet margins in this zone. As shown in FIG. 8, the heat shield members designated 115 and 116 respectively underlie the upper run of the endless bands 31 and overlie the marginal edges of the sheet of material and are spaced apart as by spacer 117 a distance slightly greater than the combined thickness of the sheet material and conveyor band. The lower heat shield is preferably grooved as indicated at 1150 in FIG. 8 to allow free passage of the protuberances during advance of the sheet 8 and the heat shields may be cooled, if desired or necessary, as by the passage of a cooling fluid through passages 11511 and 116b in the guides. Any suitable heating system can be used at the heating station 23 and in the form shown, upper and lower radiant-type heaters 23a and 23b are provided. In order to inhibit inward deflection of the conveyors 31 due to shrinkage of the plastic sheet when heated, guide bars 115C (FIG. 8) may be provided on the lower heat shields 115 and arranged to engage the inner edges of the respective conveyors.

The sheet material S after being attached to the conveyors 31 is advanced sequentially past the heating station 23 and the forming station 24 and may, as previously described, be advanced by the conveyor means past any other forming, filling or trimming stations that may be provided. The sheet margins are stripped from the conveyor bands adjacent the outlet end of the thermoforming machine and, as best shown in FIG. 9 and 10 a cam or wedge, preferably in the form of a rotary disk 118, is mounted to engage the sheet alongside the endless bands to lift the sheet off the bands as the conveyor bands are advanced. The sheet, after separation from the conveyor band, is guided as by guides 119 above the conveyors. In order to facilitate separation of the sheet margins from the conveyor bands, a heater 121 is mounted as by brackets 122 on the frame 21 of the heat sealing apparatus. The heater 121 is adjustable to a position as shown in FIG. 9 in which it closely underlies the upper run of the conveyor band in advance of the separating station, and is heated as by a heating unit 123 to heat soften the protuberances in the sheet margins. As will be readily understood, the temperature of the heater 121 can be controlled as by an adjustable thermostat (not shown) to provide the desired heat for softening the protuberances to facilitate separation of the sheet from the conveyors without causing excessive melting or distortion of the sheet.

Most of the protuberances remain with the sheet margins during separation from the conveyor bands. However, apparatus is provided for removing any protuberances that remain in the openings after separation of the sheet. This ap paratus as best shown in FIG. 10 includes a pair of opposed stripper bars 125 and 126 which are yieldably urged as by springs 127 and 128 into engagement with relatively opposite sides of the conveyor bands at a point after the separation of the sheet margins from the conveyor bands. As previously described, the protuberances are advantageously arranged to project through the conveyor bands and the stripper bars 125 and 126 are arranged to engage any projecting protuberances to strip them from the conveyor bands. A vacuum head is also provided for aiding in removal and disposal of the protuberances and, advantageously, the stripper bars can be mounted in the end of a'tubular body 129 which is slotted as indicated at 130 to allow passage of the conveyor bands 31. A vacuum pump (not shown) is connected to the tubular member 129 to draw in and collect any pieces of the sheet margin that are removed from the openings 31a.

Any suitable control arrangement may be provided for operating the sheet-attaching press 51 and the forming apparatus 24 in timed relation with the advance of the conveyor. One suitable control arrangement is diagrammatically illustrated in FIG. 11. As diagrammatically shown, the forming sta- 7 tion 24 is of the plug-assist type and includes a movable forming die or mold 131 which is moved under the control ofan ac tuator 132 into and out ofa raised position clamping the sheet to a pressure head 133. An assist plug 134 is provided in the pressure head and operated under the control of an actuator 135. The mold die actuator 132 is operated under the control of a four-way reversing valve 136 having an electroresponsive operator 136a and the assist plug actuator 135 is operated under the control of a four-way reversing valve 137 having an electroresponsive operator 137a. A two-way valve 138 is provided for controlling the application of fluid pressure to the pressure head 133 and an electroresponsive operator 1380 provided for operating valve 138. Actuators 132 and 135 can be air operated and as diagrammatically illustrated, the valves 136-138 are connected to an air supply through pressure regulators 141, 142 and 143. Relatively high pressures are required for forming the protuberances in the sheet margins and the actuator 58 is preferably hydraulically operated. As diagrammatically shown, fluid from the outlet of a pump 145 is reversibly supplied through a four-way valve 146 to the passages 68 and 78 in the fluid actuator 58 of the sheet attaching press. An electroresponsive operator 146a is provided to operate valve 146. Electroresponsive operators 136a, 137a, 138a and 1460 are conveniently operated under control of a sequence or timer controlled mechanism diagrammatically shown at 150. As shown, the timer or sequence control mechanism 150 can be operated in timed relation with the conveyor mechanism as by a switch 151 actuated by a cam 152 from the conveyor drive shaft 41.

As previously described, the conveyor drive mechanism 38 operates to advance the conveyor during one-half revolution of the drive shaft 41 and the cam 152 can be arranged to operate switch 151 during the succeeding half revolution of the drive shaft to initiate operation of the sheet attaching process control valve 146 and the forming station control valves 136, 137 and 138 during the dwell of the conveyor. As is well understood in the art, the plug assist forming apparatus operates to sequentially raise the forming die to a sheet clamping position, extend the assist head 134 and thereafter apply fluid pressure to the head 133 to press the sheet off the assist plug and into the mold 131. Thereafter, the assist plug is retracted and the forming die is lowered.

From the foregoing it is thought that the method and apparatus will be readily understood. The sheet margins are attached to the spaced endless conveyors by pressing portions of the marginal edges of the sheet into openings in spaced conveyors to permanently deform portions of the sheet into the openings and form protuberances in the sheet margins which attach the sheet to the conveyors. The protuberances retain the sheet margins on the conveyors and it is accordingly unnecessary to maintain clamping pressure on the sheet during advance through the thermoforming apparatus. The sheet margins are separated from the endless conveyors at the outlet end of the thermoforming machine by stripping the sheet margins from the sheet. With this arrangement, puncturing and rupturing of the sheet as occurred with spike-type conveyors is avoided and, moreover, it is unnecessary to maintain clamping pressure on the sheet after the sheet is attached to the conveyor and while the sheet is being advanced through the thermoforming apparatus. In addition, the endless band conveyor and the protuberances in the sheet margins which hold the sheet on the conveyors provide a firm and smooth generally continuous support for the sheet edges which inhibits wrinkling and distortion of the sheet margins during heating and thermoforming.

We claim:

1. in a thermoforming apparatus having a heating zone for heat softening a sheet of thermoplastic material, a conveying mechanism for advancing the sheet of thermoplastic material through the heating zone, said conveying mechanism including a pair of laterally spaced endless conveyor means, means for advancing one run of the endless conveyor means through the heating zone with the lateral sheet margins overlapping one side face of said one run of each of the endless conveyor means, said endless conveyor means having longitudinally spaced openings in said one side faces thereof, and sheet press means located at a sheet attaching station adjacent the inlet end of said conveying mechanism for permanently deforming longitudinally spaced portions of the sheet margins into said openings to form protuberances in the sheet margins extending into the openings in the endless conveyor means and attaching the sheet margins to the conveyor means.

2. A thermoforming apparatus according to claim 1 wherein said sheet press means includes means at the sheet attaching station for supporting said one run of the endless conveyor means and means for applying pressure to the sheet margins to press and permanently deform portions of the sheet margins into said openings.

3. A thermoforming apparatus according to claim 1 wherein said endless conveyor means each comprise an endless band having openings extending therethrough.

4. A thermoforming apparatus according to claim 3 wherein said sheet press means includes a die block having a recessed portion registering with the openings in said bands and pressure applying means for applying pressure to portions of the sheet margins to press the same through the openings in the bands and into said recesses and form protuberances which project through the bands.

5. A thermoforming apparatus according to claim 1 including means on said frame for heat softening the sheet margins prior to pressing said sheet margins into the openings in the bands.

6. In a thermoforming apparatus having a heating zone for heat softening a sheet of thermoplastic material, a conveying mechanism for advancing the sheet of thermoplastic material through the heating zone, said conveying mechanism including a pair of laterally spaced endless bands, means for advancing one run of each of the endless bands through the heating zone with said one run of the bands disposed in a common plane and with the lateral sheet margins overlapping one side face of said one run of each of said bands, said endless bands having longitudinally spaced openings extending therethrough, and sheet press means located at a sheet attaching station adjacent the inlet end of said conveying mechanism for permanently deforming portions along the lateral sheet margins into said openings to form spaced buttonlike protuberances in the sheet margins extending into the openings for anchoring the sheet margins on said one run of the endless bands.

7. A thermoforming apparatus according to claim 6 wherein said sheet press means each includes first and second die means at relatively opposite sides of one run of said bands,

means for intermittently advancing said bands, means operative while said bands are dwelling for relatively moving the first and second die means to clamp sections of the sheet margins to said one run of said bands along opposite sides of the openings in the bands, and means associated with said die means for applying pressure to the portions of the sheet overlying said openings to press'the same into the openings in the bands while the sections of the sheet margins are clamped to said bands.

8. A thermoforming apparatus according to claim 7 wherein said means for applying pressure includes a movable head in each of said first die means having an elastomeric face engageable with said portions of said sheet margins.

9. A thermoforming apparatus according to claim 8 wherein each of said second die means has a shallow recess registering with the openings in said bands, said movable head being operative to press said portions of the sheet margins through said opening and into said recess to form protuberances which project through the bands.

10. An apparatus according to claim 6 including means at a sheet separating station adjacent the outlet end of said conveying mechanism for separating the sheet margins from said endless bands, and means for removing any portions of the sheet margins that remain in said openings after the sheet margins have been separated from the endless bands.

11. An apparatus according to claim 10 wherein said means for removing portions of the sheet margins from said openings includes a vacuum head located to register with said openings as the endless bands are advanced.

12. An apparatus according to claim 10 including means in advance of said sheet separating station for heating the protuberances in the sheet margins which project into said openings in the bands to facilitate separation of the sheet margins from the bands.

13. A thermoforming apparatus according to claim 6 including guide means at each side of the sheet in the heating zone, said guide means each including a first guide member underlying the bands and a second guide member overlying the marginal edges of the sheet in the region adjacent the protuberances therein.

14. A thermoforming apparatus according to claim 13 wherein said first guide members have a channel registering with the openings in said bands to receive the protuberances on the sheet margins.

15. An apparatus for conveying thermoplastic sheet material through a heating zone comprising, a pair of laterally spaced endless conveyors having longitudinally spaced openings therein, sheet press means associated with each conveyor for attaching the sheet margins to said conveyors for advance through the heating zone, said sheet press means each including an anvil underlying a portion of each conveyor, a sheet clamp head overlying the anvil for clamping the sheet margin to the conveyor, a sheet press head movable relative to the clamp head for pressing portions of the sheet margins into the openings in the conveyor, and fluid operated means operatively associated with said sheet clamp heads and said sheet press heads for moving said clamp heads into position clamping the sheet margins to the conveyors and for thereafter moving the sheet press heads into engagement with the sheet margins to press and permanently deform portions of the sheet margins into the openings in the conveyors.

16. An apparatus according to claim 15 wherein said fluid operated means includes a movable platen, means yieldably mounting said sheet clamp head on said movable platen, and means operatively connecting said sheet press head to said movable platen for movement therewith.

17. An apparatus according to claim 15 wherein said fluid operated means includes a main cylinder in fixed overlying relation to each anvil, a movable main piston in each main cylinder defining a main piston chamber therein, each main piston having an auxiliary cylinder therein smaller than the respective main cylinder, an auxiliary piston in each auxiliary cylinder in fixedrelation to the respective main cylinder and defining an auxiliary piston chamber m each mam piston, and

means for reversibly supplying fluid pressure to said main and auxiliary piston chambers.

18. A sheet press comprising, a frame, an anvil supported on said frame, a main cylinder fixed to said frame and overlying said anvil, said cylinder having an open side facing said anvil and a main piston slidable in said main cylinder and defining a main piston chamber therein, a movable platen attached to said main piston for movement therewith, a forming die head mounted on said movable platen, said main piston having an auxiliary piston in the auxiliary cylinder fixed to the main cylinder and defining an auxiliary piston chamber in the main cylinder, and means for reversibly supplying fluid to the main and auxiliary piston chambers to respectively extend and retract the platen and forming die. I 

1. In a thermoforming apparatus having a heating zone for heat softening a sheet of thermoplastic material, a conveying mechanism for advancing the sheet of thermoplastic material through the heating zone, said conveying mechanism including a pair of laterally spaced endless conveyor means, means for advancing one run of the endless conveyor means through the heating zone with the lateral sheet margins overlapping one side face of said one run of each of the endless conveyor means, said endless conveyor means having longitudinally spaced openings in said one side faces thereof, and sheet press means located at a sheet attaching station adjacent the inlet end of said conveying mechanism for permanently deforming longitudinally spaced portions of the sheet margins into said openings to form protuberances in the sheet margins extending into the openings in the endless conveyor means and attaching the sheet margins to the conveyor means.
 2. A thermoforming apparatus according to claim 1 wherein said sheet press means includes means at the sheet attaching station for supporting said one run of the endless conveyor means and means for applying pressure to the sheet margins to press and permanently deform portions of the sheet margins into said openings.
 3. A thermoforming apparatus according to claim 1 wherein said endless conveyor means each comprise an endless band having openings extending therethrough.
 4. A thermoforming apparatus according to claim 3 wherein said sheet press means includes a die block having a recessed portion registering with the openings in said bands and pressure applying means for applying pressure to portions of the sheet margins to press the same through the openings in the bands and into said recesses and form protuberances which project through the bands.
 5. A thermoforming apparatus according to claim 1 including means on said frame for heat softening the sheet margins prior to pressing said sheet margins into the openings in the bands.
 6. In a thermoforming apparatus having a heating zone for heat softening a sheet of thermoplastic material, a conveying mechanism for advancing the sheet of thermoplastic material through the heating zone, said conveying mechanism including a pair of laterally spaced endless bands, means for advancing one run of each of the endless bands through the heating zone with said one run of the bands disposed in a common plane and with the lateral sheet margins overlapping one side face of said one run of each of said bands, said endless bands having longitudinally spaced openings extending therethrough, and sheet press means located at a sheet attaching station adjacent the inlet end of said conveying mechanism for permanently deforming portions along the lateral sheet margins into said openings to form spaced buttonlike protuberances in the sheet margins extending into the openings for anchoring the sheet margins on said one run of the endless bands.
 7. A thermoforming apparatus according to claim 6 wherein said sheet press means each includes first and second die means at relatively opposite sides of one run of said bands, means for intermittently advancing said bands, means operative while said bands are dwelling for relatively moving the first and second die means to clamp sections of the sheet margins to said one run of said bands along opposite sides of the openings in the bands, and means associated with said die means for applying pressure to the portions of the sheet overlying said openings to press the same into the openings in the bands while the sections of the sheet margins are clamped to said bands.
 8. A thermoforming apparatus according to claim 7 wherein said means for applying pressure includes a movable head in each of said first die means having an elastomeric face engageable with said portions of said sheet margins.
 9. A thermoforming apparatus according to claim 8 wherein each of said second die means has a shallow recess registering with the openings in said bands, said movable head being operative to press said portions of the sheet margins through said opening and into said recess to form protuberances which project through the bands.
 10. An apparatus according to claim 6 including means at a sheet separating station adjacent the outlet end of said conveying mechanism for separating the sheet margins from said endless bands, and means for removing any portions of the sheet margins that remain in said openings after the sheet margins have been separated from the endless bands.
 11. An apparatus according to claim 10 wherein said means for removing portions of the sheet margins from said openings includes a vacuum head located to register with said openings as the endless bands are advanced.
 12. An apparatus according to claim 10 including means in advance of said sheet separating station for heating the protuberances in the sheet margins which project into said openings in the bands to facilitate separation of the sheet margins from the bands.
 13. A thermoforming apparatus according to claim 6 including guide means at each side of the sheet in the heating zone, said guide means each including a first guide member underlying the bands and a second guide member overlying the marginal edges of the sheet in the region adjacent the protuberances therein.
 14. A thermoforming apparatus according to claim 13 wherein said first guide members have a channel registering with the openings in said bands to receive the protuberances on the sheet margins.
 15. An apparatus for conveying thermoplastic sheet material through a heating zone comprising, a pair of laterally spaced endless conveyors having longitudinally spaced openings therein, sheet press means associated with each conveyor for attaching the sheet margins to said conveyors for advance through the heating zone, said sheet press means each including an anvil underlying a portion of each conveyor, a sheet clamp head overlying the anvil for clamping the sheet margin to the conveyor, a sheet press head movable relative to the clamp head for pressing portions of the sheet margins into the openings in the conveyor, and fluid operated means operatively associated with said sheet clamp heads and said sheet press heads for moving said clamp heads into position clamping the sheet margins to the conveyors and for thereafter moving the sheet press heads into engagement with the sheet margins to press and permanently deform portions of the sheet margins into the openings in the conveyors.
 16. An apparatus according to claim 15 wherein said fluid operated means includes a movable platen, means yieldably mounting said sheet clamp head on said movable platen, and means operatively connecting said sheet press head to said movable platen for movement therewith.
 17. An apparatus according to claim 15 wherein said fluid operated means includes a main cylinder in fixed overlying relation to each anvil, a movable main piston in each main cylinder defining a main piston chamber therein, each main piston having an auxiliary cylinder therein smaller than the respective main cylinder, an auxiliary piston in each auxiliary cylinder in fixed relation to the respective main cylinder and defining an auxiliary piston chamber in each main piston, and means for reversibly supplying fluid pressure to said main and auxiliary piston chambers.
 18. A sheet press comprising, a frame, an anvil supported on said frame, a main cylinder fixed to said frame and overlying said anvil, said cylinder having an open side facing said anvil and a main piston slidable in said main cylinder and defining a main piston chaMber therein, a movable platen attached to said main piston for movement therewith, a forming die head mounted on said movable platen, said main piston having an auxiliary piston in the auxiliary cylinder fixed to the main cylinder and defining an auxiliary piston chamber in the main cylinder, and means for reversibly supplying fluid to the main and auxiliary piston chambers to respectively extend and retract the platen and forming die. 